High strength aluminum alloy sheet

ABSTRACT

A 6000-series aluminum alloy sheet used for structural materials such as structural materials and reinforcing materials increased in the strength without deteriorating bendability is provided. The strength of the Al—Mg—Si alloy sheet after BH is increased without deteriorating the bendability also after natural aging by defining the microstructure such that the content of Mg and Si are balanced so as to satisfy (Mg content)+(Si content)≧1.5% and 0.6≦(Mg content)/(Si content)≦2.0, and an exothermic peak of a predetermined height is present only by one in a temperature range of 230 to 330° C. of a differential scanning calorimeter measurement curve of the sheet shown in FIG.  1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an Al—Mg—Si alloy sheet. The aluminum alloy sheet referred to in the present invention is a rolled sheet such as a hot-rolled sheet or a cold-rolled sheet, which is an aluminum alloy sheet subjected to tempering such as solid solution treatment and quenching before being subjected to bending fabrication and paint-bake treatment. Hereinafter, aluminum may also be referred to as ALUMI or Al.

2. Description of the Related Art

Recently, social need for weight reduction of vehicles such as automobiles has increased more and more out of consideration for global environment. To meet such social need, as a material of automobiles, a lighter weight aluminum alloy material having excellent formability and paint-bake hardenability (bake hardenability, hereinafter also referred to as BH property) is used increasingly in place of steel materials such as steel sheets.

Aluminum alloy sheets for large panel materials such as outer panels and inner panels of automobiles include, for example, Al—Mg—Si alloy sheets such as AA or JIS 6000-series (also simply referred to hereinafter as 6000-series) are used. The 6000-series aluminum alloy has a composition essentially containing Si and Mg, ensures formability at a low proof stress during formation, improves the proof stress (strength) by heating during artificial aging (hardening) such as paint baking of the panel after formation, and has excellent paint-bake hardenability capable of ensuring necessary strength.

For further weight reduction of automobile bodies, extended use of an aluminum alloy materials is desired for automobile structural members such as structural materials, for example, frames and pillars or reinforcing materials such as bumper reinforcements and door beams for automobile members except for the panel material.

However, further strengthening is required for the automobile structural materials compared with the automobile panels. Accordingly, for applying the 6000-series alloy sheet used for the automobile panel materials to the structural materials or the reinforcing materials needs further strengthening.

However, it is not so easy to attain such high strengthening without greatly changing the composition and the manufacturing conditions of existent 6000-series aluminum alloy sheets and without hindering the bendability, etc.

It has been variously proposed to control an Mg—Si-based clusters as the sheet microstructure in order to improve the property such as a BH property of the 6000-series aluminum alloy sheet as the panel material. Various techniques of controlling the Mg—Si-based clusters by endothermic peaks and exothermic peaks of a differential scanning calorimeter measurement curve (also referred to as differential scanning calorimetry analysis curve, which may be also referred to as DSC) of the 6000-series aluminum alloy sheets are also proposed.

For example, JP-A 2003-27170 proposes to control a negative endothermic peak height in a temperature range of 150 to 250° C. corresponding to melting of Si vacancy cluster (GPI) in DSC after tempering including solid solution treatment and quenching of an excess Si type 6000-series aluminum alloy material to 1000 μW or less and control a positive exothermic peak height in a temperature range of 250° C. to 300° C. corresponding to precipitation of Mg/Si cluster (GPII) to 2000 μW or less. The aluminum alloy material has a proof stress of 180 MPa or more in low temperature aging of 150° C.×20 min after application of 2% strain under suppression of natural aging.

In order to obtain the BH property at low temperature for short time, Japanese Patent No. 4117243 proposes to control the exothermic peak height W1 in a temperature range of 100 to 200° C. to 50 μW or more and control the ratio W2/W1 of the exothermic peak height W2 in a temperature range of 200 to 300° C. and the exothermic peak height W1 to 20.0 or less in DSC after tempering of the 6000-series aluminum alloy sheet.

JP-A No. 2013-167004 proposes to improve the BH property (paint-bake hardening property) by selecting and controlling exothermic peak heights by the number of three (at three positions) in a predetermined temperature range in DSC particularly regarding the BH property. The three exothermic peaks include peak A in 230 to 270° C., peak B in 280 to 320° C., and peak C in 330 to 370° C. In Inventive Example 27 in Table 3 of the example (alloy No. 9 in Table 1), 0.2% proof stress after giving an artificial aging of 170° C.×20 minutes after application of 2% strain is about 259 MPa at the maximum by defining the peak B height to 20 μW/mg or more and about 31 μW/mg at the maximum, also controlling the peak ratio A/B to 0.45 or less and the ratio C/B to 0.6 or less.

SUMMARY OF THE INVENTION

In the existent control of the endothermic peaks and the exothermic peaks in DSC, since the strength before painting is controlled to a low level in order to improve the formability since the sheet is used for the panel material, the strength after BH is about less than 260 MPa at 0.2% proof stress even if the hardening amount (BH property) upon coating is increased, and the strength is insufficient as structural materials or reinforcing materials except for the panel materials.

Further, while the structural materials or reinforcing materials have no requirement for high press formability as that for the panel material, when the material sheet is fabricated into the structural material or the reinforcing material, since the material sheet is mainly subjected to bending fabrication, a bendability of such an extent as not causing cracking by V-bending fabrication is required.

The present invention has been accomplished for solving such a subject and intends to provide a high strength 6000-series aluminum alloy sheet that can be manufactured without greatly changing the composition and the manufacturing conditions of the existent 6000-series aluminum alloy sheet as the structural material or the reinforcing material, and can also be fabricated into members.

For attaining the object described above, a high strength aluminum alloy sheet of the present invention provides an Al—Mg—Si alloy sheet comprising, based on mass %, 0.6 to 2.0% of Mg, 0.6 to 2.0% of Si, 0.5% or less of Fe (not including 0%), respectively, and satisfying the conditions of (Mg content)+(Si content)≧1.5% and 0.6≦(Mg content)/(Si content)≦2.0, with the remainder comprising Al and inevitable impurities, in which an exothermic peak is present only by one and the height of the exothermic peak is within a range of 30 to 70 μW/mg in a temperature range of 230 to 330° C. of a differential scanning calorimeter measurement curve.

In the present invention, the composition and the trend of exothermic peaks in the DSC of the 6000-series aluminum alloy sheet have been reconsidered on the premise that the existent composition and manufacturing conditions of the aluminum alloy are not greatly changed. As a result, it has been found that 0.2% proof stress after BH of 185° C.×20 min can be increased to 260 MPa or more, preferably, 280 MPa or more and, more preferably, 300 MPa or more without deteriorating the bendability even after natural aging by controlling the behavior of generating precipitation phase during BH by defining the number and the height of exothermic peaks in a predetermined temperature range.

When simulating the behavior of generating the precipitation phase during BH of the 6000-series aluminum alloy sheet by DSC, exothermic peaks of precipitation of strengthened phase 1 (β′) and strengthened phase 2 (β′) are present being widely spaced apart from each other in a region of 230 to 330° C. in usual 6000-series aluminum alloy sheets used as panels for automobiles.

On the contrary, in the present invention, exothermic peaks upon precipitation of the strengthened phase 1 (β″) and the strengthened phase 2 (β′) are controlled such that the exothermic peaks of the strengthened phase 1 (β″) and the strengthened phase 2 (β′) are not separated but exothermic peaks overlap to each other by devising the composition of Mg and Si and the manufacturing conditions in a complexed manner. Thus, β′ can be formed in a great amount upon paint-bake treatment in addition to β″ and, as a result, strength after the paint-bake treatment (after BH) can be increased remarkably.

Accordingly, the present invention can satisfy the required strength of structural materials and reinforcing materials for automobiles which are stronger than automobile panel materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a DSC of a typical example in a preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are to be described specifically on each of requirements.

(Chemical Composition)

First, a chemical composition of an Al—Mg—Si based (hereinafter also referred to as 6000-series) aluminum alloy sheet of the present invention is to be described below. In the present invention, structural materials or reinforcing materials except for the panel materials are strengthened without deteriorating the bendability and without greatly changing the existent composition and the manufacturing conditions.

In order to satisfy such subjects in view of the composition, the 6000-series aluminum alloy sheet has a composition comprising, based on mass %, 0.6 to 2.0% of Mg, 0.6 to 2.0% of Si, 0.5% or less of Fe (not including 0%) respectively, and satisfying relations: (Mg content)+(Si content)≧1.5% and 0.6≦(Mg content)/(Si content)≦2.0, with the remainder consisting of Al and inevitable impurities. “%” for the content of each of the elements always means mass %.

The range of content and the meaning of each element or the allowable amount thereof in the 6000-series aluminum alloy are to be described.

Si: 0.6 to 2.0%

Si, together with Mg, is an essential element for obtaining a necessary strength (proof stress) as the structural material or the reinforcing material described above by forming aging precipitates that contribute to the improvement of the strength upon artificial aging such as paint-bake treatment, thereby providing artificial age hardenability. If the Si content is insufficient, the amount of aging precipitates after artificial aging is insufficient to lower the strength. In contrast, if the Mg content is excessive, hot rolling cracks tend to occur during manufacture of the sheet. Further, coarse constituents and precipitates are formed to remarkably deteriorate the bendability. Accordingly, the Si content is defined within a range from 0.6 to 2.0%.

Mg: 0.6-2.0%

Mg, together with Si, is also an essential element for obtaining a high strength (proof stress) by forming aging precipitates that contribute to the improvement of the strength thereby providing artificial age hardenability. If the Mg content is insufficient, the amount of aging precipitates after artificial aging is insufficient to lower the strength. In contrast, if the Mg content is excessive, hot rolling cracks tend to occur during manufacture of the sheet. Further, coarse constituents and precipitates are formed to remarkably deteriorate the bendability. Accordingly, the Mg content is defined within a range from 0.6 to 2.0%.

(Mg content)+(Si content)≧1.5%

0.6≦(Mg content)/(Si content)≦2.0

Both of (Mg content)+(Si content) as the total contents of Mg and Si, and (Mg content)/(Si content) as the content ratio of Mg to Si greatly affect on the number and the height of the exothermic peaks as the microstructure of the 6000-series aluminum alloy sheet in a temperature range of 230 to 330° C. corresponding to precipitation of Mg/Si cluster (GPII) in the differential scanning calorimeter measurement curve.

By defining (Mg content)+(Si content) to 1.5% or more and defining (Mg content)/(Si content) to a range of 0.6 to 2.0 on the premise of using an appropriate manufacturing method to be described later, the exothermic peak can be present only by one in the temperature range of 230 to 330° C. and the height of the exothermic peak can be in a range of 30 to 70 μW/mg.

Accordingly, (Mg content)+(Si content) is 1.5% or more and, preferably, as large as possible. If (Mg content)+(Si content) is less than 1.5%, the exothermic peak present in the temperature range of 230 to 330° C. cannot be restricted to only one, or the exothermic peak height cannot be within a range of 30 to 70 μW/mg. Accordingly, the strength after the natural aging and after BH cannot be increased to at least 260 MPa or more, preferably, 280 MPa or more and, more preferably, 300 MPa or more.

The upper limit of (Mg content)+(Si content) is determined by the amount of limit that the sheet described above can be manufactured without causing cracks and the upper limit of (Mg content)+(Si content) is preferably 2.5%.

(Mg content)/(Si content) is 2.0 or less and, preferably, as small as possible. If it is too large and more than 2.0, it becomes difficult to control the exothermic peak of DSC within the defined range even by adopting the appropriate manufacturing conditions to be described later. That is, the exothermic peak present in the temperature range of 230 to 330° C. cannot be restricted to only one or the exothermic peak height cannot be restricted to the range of 30 to 70 μW/mg. Accordingly, the strength after the natural aging and after BH cannot be increased to at least to 260 MPa or more, preferably, 280 MPa or more and, more preferably, 300 MPa or more.

Since the lower limit of (Mg content)/(Si content) is determined by the limit that the sheet described above can be manufactured without causing hot rolling cracks, the lower limit of (Mg content)/(Si content) is 0.6.

Fe: 0.5% or less (not including 0%)

Fe serves to form constituents as nuclei for recrystallized grains thereby inhibiting crystal grains from coarsening. Accordingly, Fe is contained preferably by 0.05% or more. If Fe is more than 0.5%, coarse compounds are formed as fracture origins to deteriorate the strength or the bendability. Accordingly, the Fe content is defined as 0.5% or less (not including 0%).

Other Elements

In addition, in the present invention, for increasing the strength of the aluminum alloy sheet, one or more of 0.5% or less of Mn (not including 0%), 0.3% or less of Cr (not including 0%), 0.1% or less of Zr (not including 0%), 0.1% or less of V (not including 0%), 0.1% or less of Ti (not including 0%), 1.0% or less of Cu (not including 0%), 0.1% or less of Ag (not including 0%), 0.30% or less of Zn (not including 0%), and 0.005 to 0.15% of Sn may also be contained.

Since the elements have an effect of increasing the strength of the sheet in common, they can be regarded as elements of providing equivalent strengthening effect but their specific mechanisms, naturally, include common portions and different portions.

Mn, Cr, Zr, and V form dispersed particles (dispersion phase) during the homogenizing heat treatment and such dispersed particles have an effect of hindering grain boundary movement after recrystallization and serve to refine crystal grains to increase the strength of the sheet. Further, Ti serves to form constituents as nuclei of recrystalized grains thereby suppressing crystal grains from coarsening, and refining crystal grains to increase the strength of the sheet. Cu, Zn, and Ag are useful for improving the artificial aging hardenability (BH property) and has an effect of promoting precipitation of a compound phase such as a GP zone into the crystal grains of the sheet microstructure thereby increase the strength of the sheet. Sn has an effect of suppressing diffusion of Mg and Si by capturing atom vacancy thereby suppressing the increase of strength at a room temperature (natural aging), and releasing the captured vacancy during artificial aging and promoting diffusion of Mg and Si, thereby increasing the BH property to increase the strength of the sheet.

However, if the content of each of the elements is excessive, coarse compounds are formed, etc., making the manufacture of the sheet difficult and also deteriorating the strengthen and the bendability. Further, corrosion resistance is also deteriorated. Accordingly, if the elements are contained, their contents are restricted to each of the upper limit values or less as described above.

(Differential Scanning Calorimeter Measurement Curve, Differential Scanning Calorimetry Analysis Curve, DSC)

In order to ensure high strength as the structural material or the reinforcing material of automobile members, etc., on the premise of the composition as described above, the exothermic peak is present only by one in the temperature range of 230 to 330° C. in the differential scanning calorimeter measurement curve of the sheet and the height of the exothermic peak is defined in a range of 30 to 70 μW/mg, preferably, in a range of 35 to 70 μW/mg in the present invention.

Thus, 0.2% proof stress after BH (after paint-bake treatment) of 185° C.×20 minutes can be increased to 260 MPa or more, preferably, 280 MPa or more and, more preferably, 300 MPa or more without greatly changing the existent composition and the manufacturing conditions and without deteriorating the bendability of the aluminum alloy.

The differential scanning calorimeter measurement curve (DSC) is a heating curve from a solid phase obtained by measuring the thermal change in the melting process of an aluminum alloy sheet after tempering by differential scanning calorimeter measurement under the conditions to be described later. Accordingly, the behavior of generating the precipitation phase of 6000-series aluminum alloy sheet during BH can be accurately reflected or simulated by DSC.

More specifically, in a 6000-series aluminum alloy, various precipitation phases are formed such as a cluster, a GP zone, a strengthened phase 1 (β″), a strengthened phase 2 (β′), an equilibrium phase (Mg₂Si), etc. depending on the artificial aging temperature. For increasing the strength after paint-bake treatment (artificial aging), it is effective to form, particularly, the strengthened phase 1 (β″) and the strengthened phase (β′), during paint-bake treatment. Then, the change of the behavior of generating β″ and β′ during BH (paint-bake treatment) can be simulated by the DSC, and this provides the basis of defining the microstructure by DSC in the present invention.

In the manufacture of usual 6000-series aluminum alloy sheets used as the panels for the automobiles by the ordinary method, the exothermic peaks of the strengthened phase 1 (β″) and the strengthened phase 2 (β′) in DSC are present being spaced widely from each other in a range of 230 to 330° C. That is, many exothermic peaks of β″ were present so far near the low temperature former half in 240 to 260° C. of the temperature range, whereas existent exothermic peaks of β′ were present near the low temperature latter-half in 280 to 300° C. of the temperature range. Comparative Example 11 in Table 2 of the example illustrated in FIG. 1 to be described later is a typical example.

When two or more exothermic peaks are present individually (independently or separately) in a range of 230 to 330° C. as in the 6000-series aluminum alloy sheet for automobile panels, this means that an absolute (total) amount of the substantial strengthened phase (precipitation amount) during BH is decreased since the strengthened phase 1 (β″) and the strengthened phase 2 (β′) are present being dispersed. In other words, this means that there is a limit of increasing (making higher) the height of each of exothermic peaks of the strengthened phase 1 (β″) and the strengthened phase 2(β′) in DSC.

In addition, it is important that even when the height of the exothermic peak of the β″ present near 240 to 260° C. is lowered and the exothermic peak height of β′ present near 310 to 320° C. is increased or the height of each of the exothermic peaks of β″ or β′ in DSC is increased as in JP-A 2013-167004, this does not increase the strength by so much for the height of the exothermic peak. That is, even if the exothermic peaks are high, they do not provide high strength required for structural materials or reinforcing materials of automobiles for which higher strength is required than that for the panel materials.

On the contrary, it has been found that when the conditions of pre-aging after solid solution treatment and quenching in the tempering after rolling the sheet by changing the manufacturing method together with the change for the composition, the exothermic peaks of β″ and β′ are generated such that temperature difference between each of the peaks is decreased and peaks overlap each other.

According to the finding of the present inventors, the generation temperature of the exothermic peak of β″ (also referred to as first peak or former-half peak) shifts from the lower temperature position (temperature) so far to a position (temperature) near 270 to 290° C. at higher temperature. The generation temperature of the other exothermic peak β′ (also referred to as second peak or latter-half peak) shifts from the higher temperature position (temperature) so far to a position (temperature) near 290 to 300° C. at lower temperature.

As described above, when the temperature difference between each of the peaks is decreased in the exothermic peaks β″ and β′ and they are generated being overlapped and synthesized at the peaks, an amount of artificial aging precipitates of increasing the proof stress after BH can be ensured. That is, in the present invention, the exothermic peaks upon precipitation of the strengthened phase 1 (β″) and the strengthened phase 2 (β′) are controlled such that they are not separated but overlapped at the exothermic peaks each other by devising the composition of Mg and Si and the manufacturing conditions in a complexed manner. Thus, also β′ can be formed in a great amount in addition to β″ upon paint-bake treatment (BH) and the absolute amount of β″ and β′ formed (precipitation amount) can be increased to increase the strength after the paint-bake treatment (after BH) as far as high strength required for the structural materials or the reinforcing materials of the automobiles.

The fact that the exothermic peak is present only by one and the exothermic peak is high in the temperature range of 230 to 330° C. of the DSC means that β″ and β′ are formed in great amounts during differential scanning calorimeter measurement or during the paint-bake treatment (artificial aging) simulated thereby and means that clusters as nuclei for β″ and β′ are small at the time before paint-bake treatment.

That is, if the exothermic peak is excessively high, formation of the cluster as the nuclei for β″ and β′ is insufficient at the time before paint-bake treatment (before artificial aging) and the strength before paint-bake treatment is lowered, so that the strength after coating cannot be increased. Accordingly, the height of the single exothermic peak in the temperature range of 230 to 330° C. is defined as 70 μW/mg or less.

On the other hand, if the exothermic peak is excessively low, this means that the amount of β″ or β′ formed during the differential scanning calorimeter measurement or during the paint-bake treatment (artificial aging) simulated thereby is small. That is, β″ and β′ and clusters as nuclei therefor are formed excessively at the time before paint-bake treatment (artificial aging), so that the BH amount after paint-bake treatment is decreased and the strength is increased excessively during bending fabrication before paint-bake treatment to deteriorate the bendability. Accordingly, the height of the single exothermic peak within the temperature range of 230 to 330° C. is defined as 30 μW/mg or more and, preferably, 35 μW/mg or more.

As described above, the exothermic peaks in the differential scanning calorimeter measurement curve defined in the present invention can be obtained by performing pre-aging at high temperature for a long time as will be described later in a state where Mg and Si are sufficiently solid solutionized after solid solution treatment and quenching of a cold rolled sheet.

Manufacturing Method

Then, a method of manufacturing the aluminum alloy sheet of the invention is to be described. The manufacturing process per se of the aluminum alloy sheet of the invention is a customary or known process, in which an aluminum alloy slab having the 6000-series composition is cast and then subjected to a homogenizing heat treatment, hot-rolled and cold-rolled into a sheet of a predetermined thickness and further subjected to tempering such as solid solution treatment and quenching.

In the manufacturing process, in order to obtain the microstructure defined by the DSC of the present invention, the conditions of the pre-aging after solid solution treatment and quenching are defined within the preferred range as will be described later. Also other processes include preferred conditions for obtaining the microstructure defined by DSC of the present invention. Without such preferred conditions, it is difficult to obtain the microstructure defined by the DSC in the present invention.

(Melting and Casting Cooling Rate)

First, in the melting and casting process, a molten aluminum alloy melted and adjusted within the range of the 6000-series composition is cast by properly selecting usual melting and casting process such as a continuous casting, a semi-continuous casting method (DC casting), etc. For controlling the clusters within the range defined in the present invention, an average cooling rate during casting is preferably as high (fast) as possible from the liquidus temperature to the solidus temperature, for example, at 30° C./min or higher.

Without such temperature control (cooling rate) in a high temperature region during casting, the cooling rate in the high temperature region is inevitably lowered. If the average cooling rate in the high temperature region is lowered, the amount of constituents formed coarsely in the temperature range in the high temperature region is increased, and the size and the amount of the constituents vary greatly in the direction of the width and in the direction of the thickness of the slab. As a result, there may be a high possibility that clusters cannot be controlled as defined in the range of the present invention.

(Homogenizing Heat Treatment)

Subsequently, the cast aluminum alloy slab is subjected to a homogenizing heat treatment prior to hot rolling. The homogenizing heat treatment (soaking) is important for sufficiently solid-solutionizing Si and Mg in addition to homogenization of the microstructure (eliminating segregation in the crystal grains in the slab microstructure) as an ordinary purpose. So long as the purpose is attained under the conditions, the conditions are not particularly restricted and the treatment is usually applied for once or in one step.

Si and Mg are solid-solutionized sufficiently by properly selecting the homogenizing heat treatment temperature from the range of 500° C. or higher and 560° C. or lower and the homogenizing (holding) time from the range of one hour or more. If the homogenizing temperature is lower, the solid solution amount of Si and Mg cannot be ensured and the exothermic peak of the DSC cannot be defined as described above even by the pre-aging (reheating) after the solid solution treatment and the quenching to be described later. Further, segregation in the crystal grains cannot be eliminated sufficiently and since the segregation acts as fracture origins, bendability is deteriorated.

After the homogenizing heat treatment, hot rolling is performed at 450° C. or higher and it is necessary not to lower the temperature of the slab to 500° C. or lower till starting of hot rough rolling after the homogenizing heat treatment thereby ensuring the solid solutionized amount of Si and Mg. If the temperature of the slab is lowered to 550° C. or lower till starting of rough rolling, there may be a high possibility that Si and Mg are precipitated failing to ensure the solid solutionized amount of Si and Mg for defining the exothermic peaks of the DSC described above.

(Hot Rolling)

Hot rolling includes a rough rolling step of a slab and a finish rolling step depending on the thickness of the sheet to be rolled. In the rough rolling step or finish rolling step, reversed type, tandem type rolling mill, etc. are used properly.

During rolling from the start to the end of hot rough rolling, it is necessary not to lower the temperature to 450° C. or lower thereby ensuring the solid solution amount of Si and Mg. For this purpose, the hot rough rolling may be performed preferably with the lowest temperature of the rough rolling sheet between passes of hot rough rolling is 450° C. or higher. If the temperature of the rough rolling sheet is lowered to 450° C. or lower during hot rough rolling, there may be a high possibility that Si and Mg are precipitated failing to ensure the solid solution amount of Si and Mg in order to define the exothermic peaks of the DSC described above.

After the hot rough rolling described above, a hot finish rolling with an end temperature in a range of 300 to 360° C. is performed. If the soaking temperature or the end temperature of the finish rolling is too low, Mg and Si type compounds are formed during soaking and hot rolling in which the balance of solid solutionized Mg/Si is changed compared with Mg/Si of Mg, Si composition upon addition, tending to form two or more exothermic peaks in the temperature range of 230 to 330° C., making it difficult that the strength after paint-bake treatment cannot be increased to a desired value.

(Annealing of Hot Rolled Sheet)

Annealing before cold rolling of the hot rolled sheet (rough annealing) may be applied although this is not always necessary.

(Cold Rolling)

In the cold rolling, the hot rolled sheet is rolled to manufacture a cold rolled sheet (also including coil) of a desired sheet thickness. For refining the crystal grains further, the cold rolling compression reduction is preferably 60% or more, and intermediate annealing may be performed between cold rolling passes with the same purpose as that for rough annealing.

(Solid Solution Treatment and Quenching)

After cold rolling, solid solution treatment and successive quenching to a room temperature are performed. For the solid solution treatment and quenching, usual continuous heat treatment line may be used. However, since it is desired to obtain a sufficient solid solutionized amount of each of elements such as Mg and Si and it is desired that the crystal grain is finer, the treatment is preferably performed under the conditions of heating to the solid solution temperature at 520° C. or higher and the melting temperature or lower at a heating rate of 5° C./sec or more and possessing for 0.1 to 20 seconds.

With a view point of suppressing formation of coarse grain boundary compounds that may deteriorate the bendability, it is preferred to control the average cooling rate from the solid solution temperature to the quenching stop temperature at a room temperature (average cooling rate) to 20° C./s or more. If the average cooling rate for the quenching to the room temperature after solid solution treatment is small, coarse Mg₂Si and elemental Si are formed during cooling to deteriorate the bendability. Further, the solid solutionized amount after solid solution treatment is lowered to deteriorate the BH property. In order to ensure the cooling rate, air cooling means such as blower, water cooling means such as mist, spray or dipping, as well as conditions therefor are selected and used respectively for the quenching.

(Pre-Aging: Reheating)

After cooling to a room temperature by quenching after the solid solution treatment, the cold rolled sheet is subjected to pre-aging (reheating) within one hour. If the room temperature holding time after the end of the quenching to the room temperature to the starting of the pre-aging (starting of heating) is excessively long, clusters liable to be solved are formed by natural aging in which the exothermic peaks defined by the DSC in the present invention are less formed. Accordingly, it is preferred that the room temperature holding time is as short as possible, and the solid solution treatment and the quenching may be in continuous with the reheating with no time interval. The lower limit for the time is not particularly defined.

In the pre-aging, the sheet is held at 60 to 120° C. for a holding time of 10 hours or more and 40 hours or less. Thus, the exothermic peaks defined by the DSC in the present invention are formed.

If the pre-aging temperature is lower than 60° C., or the holding time is less than 10 hours, the result of the treatment is identical with that in a case without the pre-aging, in which precipitated nuclei are not formed sufficiently, the peak height of the exothermic peak in the temperature range of 230 to 330° C. is increased to more than 70 μW/mg tending to lower the proof stress after the paint-bake treatment.

On the other hand, if the temperature is higher than 120° C. or holding time is longer than 40 hours, in the pre-aging conditions, the amount of precipitated nuclei formed is excessive in which the peak height of the exothermic peak in the temperature range of 230 to 330° C. is less than 30 μW/mg in the differential scanning calorimeter measurement curve and, as a result, the strength upon bending fabrication before paint-bake treatment is excessively high tending to deteriorate the bendability.

The present invention is to be described more specifically with reference to examples but it will be apparent that the present invention no way undergoes restriction by the following examples and can be practiced with appropriate modification in a range adaptable to the gists of the invention as described before and to be described later, any of which is included within the technical scope of the present invention.

EXAMPLE

6000-series aluminum alloy sheets of different microstructures defined by the DSC in the present invention were separately fabricated by changing composition and manufacturing conditions, and As proof stress, BH property (paint-bake hardenability), and bendability were measured and evaluated respectively after holding at a room temperature for 100 days after manufacture of the sheets. The result is shown in Tables 1 and 2.

Specifically, 6000-series aluminum alloy sheets having compositions as shown in Table 1 were manufactured separately while variously changing the conditions, for example, the soaking temperature, the lowest temperature of the roughly rolled sheet between passes of hot rough rolling (described as the lowest temperature in Table 2), and the hot finish rolling end the temperature and temperature and the holding time for the pre-aging. In the expression for the content of each of the elements in Table 1, columns for respective elements which are left blank with no numerical values indicate that the contents are below the detection limit.

Specific manufacturing conditions of the aluminum alloy sheets are as described below. Aluminum alloy slabs of respective compositions shown in Table 1 were melted in common by a DC casting method. In this case, an average cooling rate during casting from a liquidus temperature to a solidus temperature was defined as 50° C./min in common with each of examples. Successively, after subjecting the slabs to soaking for 6 hours in common with each of examples under the temperature conditions shown in Table 2, hot rough rolling was started at that temperature. Table 2 also shows the lowest (pass) temperature in the hot rough rolling.

Then, in common with each of the examples, the sheets were hot rolled in the succeeding finish rolling at an end temperature shown in Table 2 to a thickness of 5.0 mm to form hot rolled sheets. After subjecting the aluminum alloy sheets after hot rolling to rough annealing of 500° C.×1 minute in common with each of the examples, the sheets were subjected to cold rolling at a compression reduction of 60% with no intermediate annealing in the course of cold rolling pass to obtain cold rolled sheets of 2.0 mm thickness.

Each of the cold rolled sheets was continuously subjected to tempering (T4) while being recoiled and coiled in a continuous heat treatment facility in common with each of the examples. Specifically, solid solution treatment was performed at an average heating rate of 10° C./sec up to 500° C. and holding the sheet for five seconds after reaching the aimed temperature of 540° C., and then the sheet was cooled to a room temperature by water cooling at an average cooling rate of 100° C./sec. Just after the cooling, pre-aging was performed at temperature (° C.) and for holding time (hr) shown in Table 2. After the pre-aging, gradual cooling (spontaneous cooling) was performed.

Sheet specimens (blank) were cut out from respective final sheet products after being left for 100 hours at a room temperature after the tempering and each of the sheet specimens was measured and evaluated for the DSC and the properties. The result is shown in Table 2.

(DSC)

The DSC of the microstructure was measured for ten points at a central portion for the thickness of each sheet specimen and exothermic peaks present in a temperature range of 230 to 330° C. were measured for the DSC (differential scanning calorimeter measurement curve) of the sheet as an average value for the ten points. In this case, when there were two exothermic peaks in the temperature range, peak heights (μW/mg) of the respective exothermic peaks were determined.

The differential scanning calorimeter measurement was performed at each of measuring points of the sheet specimen by using DSC 220G manufactured by Seiko Instruments Co. as a test apparatus, a standard specimen made of aluminum, a sample vessel made of aluminum, under identical conditions of a temperature elevation rate of 15° C./min, in an atmosphere of argon (50 ml/min), and a sample weight of 24.5 to 26.5 mg respectively. The obtained profile of the differential scanning calorimeter measurement (μW) was divided by the sample weight (μW/mg) and normalized. Then, a region where the profile of the differential scanning calorimeter measurement was horizontal in a section of 0 to 100° C. was defined as a reference level of 0 and the height of the exothermic peak from the reference level was measured. The result is shown in Table 2.

(Paint-Bake Hardenability)

0.2% proof stress (As proof stress) was determined as mechanical properties of the sheet specimen by a tensile test. After 2% stretch that simulated bending fabrication for each of the sheet specimens in common and, after artificial aging for 185° C.×20 minutes (after BH), 0.2% proof stress (proof stress after BH) of the sheet specimens was determined by a tensile test. The BH property of the respective sheet specimens was evaluated based on the difference between 0.2% proof stress each other (increment of the proof stress).

In the tensile test, No. 5 test specimen of JIS Z 2201 (25 mm×50 mmGL×sheet thickness) was sampled from each of the sheet specimens and subjected to the tensile test at a room temperature. The tensile direction of the test specimen was in a direction orthogonal to the rolling direction. The tensile speed was set to 5 mm/min up to the 0.2% proof stress and to 20 mm/min after applying the proof stress. The number of times N of the mechanical property measurement was 5 and each of result was calculated as an average value. For the test specimen for measuring the proof stress after the BH. The BH treatment was performed after applying 2% preliminary strain that simulated the press forming of the sheet by a tensile tester.

(Bendability)

Bendability was measured for each of sheet specimens. In the test, a test specimen of 30 mm width×35 mm length was prepared while taking a major axis in the rolling direction, and 90° V-bending was performed at a bending radius of 2.0 mm while applying a load of 2000 kgf according to JIS Z 2248.

The surface states of the V-bent portion, for example, generation of roughening, fine cracks, and large cracks were observed visually and evaluated visually on the following criteria of 9 to 1 ranks and those of larger numerical values were evaluated as having good bendability and those of 6 or more numerical values were evaluated as satisfactory.

-   9: no cracks, no roughening, -   8: no cracks, slight roughening, -   7: no cracks, but with roughening, -   6: slight fine cracks, -   5: fine cracks, -   4: fine cracks over the entire surface, -   3: large cracks, -   2: large cracks, about to be ruptured, -   1: ruptured.

As shown in Tables 1 and 2 respectively, inventive examples 1 to 8 are manufactured within the range of the chemical composition and within the range of preferred conditions of the present invention. Accordingly, in each of the inventive examples, as shown in Table 2, the exothermic peak is present only by one in the temperature range of 230 to 330° C. as defined in the present invention and the height of the exothermic peak is in the determined range in the DSC.

As a result, each of the inventive examples is excellent in the BH property and excellent in the bendability also after natural aging as shown in Table 2.

On the contrary, Comparative Examples 1 to 5 in Table 2 use an alloy example 1 identical with that of the inventive example in Table 1. However, in each of the comparative examples, as shown in Table 2, manufacturing conditions such as soaking temperature, the lowest temperature in the hot rough rolling, the end temperature of the hot finish rolling, the temperature and the holding time of the pre-aging are out of preferred conditions. As a result, the DSC is out of the range defined in the present invention and, when compared with the inventive example 1 using the identical alloy composition, one or both of the BH property after the natural aging and the bendability is deteriorated.

Among them, in Comparative Example 1, the soaking temperature, the lowest temperature in hot rough rolling, the end temperature of the hot finish rolling, etc. are excessively low. Accordingly, exothermic peaks are formed by two in the temperature range of 230 to 330° C., the BH property was relatively low and 0.2% proof stress after BH is insufficient.

In Comparative Example 2, the temperature for the pre-aging is excessively low. Accordingly, while the exothermic peak is present by one in the temperature range of 230 to 330° C., the peak height is excessively high and the 0.2% proof stress after BH is insufficient.

In Comparative Example 3, the temperature for the pre-aging is excessively high. Accordingly, while the exothermic peak is present by one in the temperature range of 230 to 330° C., the peak height is excessively low and while 0.2% proof stress after BH is high, the bendability is lowered excessively.

In Comparative Example 4, the holding time in the pre-aging is excessively short. Accordingly, while exothermic peak is present by one in the temperature range of 230 to 330° C., the peak height is excessively high and the 0.2% proof stress after BH is insufficient.

In Comparative Example 5, the holding time in the pre-aging is excessively long. Accordingly, while exothermic peak is present by one in the temperature range of 230 to 330° C., the peak height is excessively low and while 0.2% proof stress after BH is high, the bendability is insufficient.

While Comparative Examples 6 to 13 in Table 2 are manufactured within a preferred range also including the conditions for the pre-aging, alloys of Nos. 10 to 17 in Table 1 are used respectively and the alloy compositions are out of the range of the present invention respectively.

Accordingly, in the comparative examples described above, as shown in Table 2, the DSC, etc. are out of the range defined in the present invention and one or both of the BH property after the natural aging and the bendability are deteriorated compared with the inventive examples.

In Comparative Example 6, alloy 10 in Table 1 is used in which Mg is insufficient, and (Mg content)/(Si content) is out of the lower limit. Accordingly, two exothermic peaks are formed within the temperature range of 230 to 330° C. and the BH property is poor compared with that of the inventive examples.

In Comparative Example 7, alloy 11 in Table 1 is used in which Si is insufficient and (Mg content)/(Si content) is out of the upper limit. Accordingly, two exothermic peaks are formed in the temperature range of 230 to 330° C. and the BH property is poor compared with that of the inventive examples.

In Comparative Example 8, alloy 12 in Table 1 is used in which Si is excessive. Accordingly, hot rolling cracks were generated and rolled sheet could not be manufactured.

In Comparative Example 9, alloy 13 in Table 1 is used in which (Mg content)+(Si content) is out of the lower limit. Accordingly, the BH property is poor compared with that of the inventive examples.

In Comparative Example 10, alloy 14 in Table 1 is used in which (Mg content)/(Si content) is out of the lower limit. Accordingly, two exothermic peaks are formed in the temperature range of 230 to 330° C. and the BH property is poor compared with that of the inventive examples.

In Comparative Example 11, alloy 15 in Table 1 is used in which the Mg content and (Mg content)/(Si content) are out of the lower limit. Accordingly, two exothermic peaks are formed in the temperature range of 230 to 330° C., and the BH property is poor compared with that of the inventive examples.

In Comparative Example 12, alloy 16 in Table 1 is used in which the Fe content is excessive and more than the upper limit. Accordingly, the BH property and the bendability are poor compared with those of the inventive examples.

In Comparative Example 13, alloy 17 in Table 1 is used in which the Mg content is excessive and more than the upper limit. Accordingly, the BH property and the bendability are poor compared with those of the inventive examples.

DSCs selected from the inventive examples and the comparative examples are shown in FIG. 1. In FIG. 1, a fat solid line represents Inventive Example 2 in Table 2, a dotted line represents Comparative Example 11 in Table 2, and a broken line (chained line) represents Comparative Example 6 in Table 2.

The result of the examples described above supports that all the compositions and each of the conditions for the DSC defined in the present invention should be satisfied in order to increase the 0.2% proof stress after BH of 185° C.×20 minutes without deteriorating the bendability also after natural aging to 260 MPa or more, preferably, 280 MPa or more and, more preferably, 300 MPa or more.

TABLE 1 Alloy Chemical composition of Al—Mg—Si alloy sheet (mass %, remainder Al) No. Mg Si Mg + Si Mg/Si Sn Fe Mn Cu Cr Zr V Ti Zn Ag 1 0.78 1.03 1.81 0.76 — 0.18 2 0.80 1.04 1.84 0.77 0.07 0.18 3 0.68 1.11 1.79 0.62 0.24 0.1 0.05 4 1.02 0.67 1.69 1.52 0.21 0.7 5 1.28 1.02 2.30 1.26 0.22 0.05 0.1 0.1 6 0.88 1.33 2.21 0.66 0.24 0.05 0.15 7 0.65 0.91 1.56 0.71 0.19 0.07 0.2 9 0.78 0.99 1.77 0.78 0.02 0.19 0.3 0.2 10 0.54 0.99 1.52 0.55 0.24 11 1.20 0.52 1.71 2.31 0.21 12 0.62 2.11 2.73 0.29 0.23 13 0.64 0.64 1.28 1.01 0.22 14 0.61 1.25 1.86 0.49 0.19 15 0.42 1.23 1.65 0.34 0.05 0.21 16 0.78 1.03 1.81 0.76 0.77 17 0.78 1.03 1.80 0.76 0.24 1.1

TABLE 2 Aluminum alloy sheet after holding at room temperature for 100 days Manufacturing conditions Differential scanning calorimeter of aluminum alloy sheet measurement curve Exothermic Hot peak at 230 to 330° C. rough Hot finish Exo- Soaking rolling rolling Exo- thermic Soaking Lowest end Pre-aging Exo- thermic peak temper- temper- temper- Temper- thermic peak temper- Alloy No. ature ature ature ature Time peak height ature Section No. in Table 1 ° C. ° C. ° C. ° C. hr Number μW/mg ° C. Inventive Example 1 1 540 490 330 100 15 1 58.7 283 Comparative Example 1 1 480 420 280 100 15 2 15.7 247 Comparative Example 2 1 540 490 330 50 15 1 81.6 292 Comparative Example 3 1 540 490 330 130 15 1 12.6 271 Comparative Example 4 1 540 490 330 100 5 1 74.8 288 Comparative Example 5 1 540 490 330 100 60 1 15.3 272 Inventive Example 2 2 560 500 350 100 15 1 50.3 275 Inventive Example 3 3 540 490 310 90 25 1 51.6 282 Inventive Example 4 4 540 490 310 80 30 1 35.6 291 Inventive Example 5 5 540 490 320 90 12 1 48.3 293 Inventive Example 6 6 540 490 330 100 15 1 61.5 281 Inventive Example 7 7 520 480 340 110 10 1 62.3 283 Inventive Example 9 9 550 500 360 70 35 1 53.6 276 Comparative Example 6 10 540 490 330 100 15 2 15.9 246 Comparative Example 7 11 540 490 350 100 15 2 37.2 268 Comparative Example 8 12 540 490 350 Comparative Example 9 13 540 490 330 100 15 1 31.2 287 Comparative Example 10 14 540 490 330 100 15 2 16.5 248 Comparative Example 11 15 540 490 330 100 15 2 40.1 251 Comparative Example 12 16 540 490 330 100 15 1 31.1 286 Comparative Example 13 17 540 490 350 100 15 1 36.5 288 Aluminum alloy sheet after holding at room temperature for 100 days Differential scanning calorimeter measurement curve Exothermic peak at 230 to 330° C. 2nd exo- Property 2nd exo- thermic As proof 0.2% Proof thermic peak stress proof stress peak temper- 0.2% proof stress increment Bendability height ature stress after BH in BH 90° Section No. μW/mg ° C. MPa MPa MPa V-bending Inventive Example 1 — — 178 291 113 7 Comparative Example 1 34.3 300 156 244 88 8 Comparative Example 2 — — 146 217 71 9 Comparative Example 3 — — 221 294 73 5 Comparative Example 4 — — 162 256 94 9 Comparative Example 5 — — 210 298 88 5 Inventive Example 2 — — 166 285 119 9 Inventive Example 3 — — 155 278 123 9 Inventive Example 4 — — 184 266 82 7 Inventive Example 5 — — 148 267 119 7 Inventive Example 6 — — 193 315 122 9 Inventive Example 7 — — 173 278 105 7 Inventive Example 9 — — 167 301 134 9 Comparative Example 6 34.5 300 146 248 102 9 Comparative Example 7 15.5 308 104 183 79 9 Comparative Example 8 Comparative Example 9 — — 107 187 80 9 Comparative Example 10 32.7 298 147 258 111 8 Comparative Example 11 37.1 299 108 226 118 9 Comparative Example 12 — — 153 230 77 4 Comparative Example 13 — — 184 265 81 5

According to the present invention, a 6000-series aluminum alloy sheet increased in the strength without deteriorating bendability can be provided. As a result, application use of the 6000-series aluminum alloy sheet can be extended as automobile structural materials except for panel materials, for example, structural materials such as frames and pillars or reinforcing materials such as bumper reinforcements and door beams. 

What is claimed is:
 1. A high strength Al—Mg—Si alloy sheet comprising, based on mass %, 0.6 to 2.0% of Mg, 0.6 to 2.0% of Si, and 0.5% or less (not including 0%) of Fe respectively, and satisfying (Mg content)+(Si content)≧1.5% and 0.6≦(Mg content)/(Si content)≦2.0, with the remainder consisting of Al and inevitable impurities, wherein an exothermic peak is present only by one in a temperature range of 230 to 330° C. and the height of the exothermic peak is in a range of 30 to 70 μW/mg in a differential scanning calorimeter measurement curve of the sheet, providing that the differential scanning calorimeter measurement is conducted by using DSC 220G manufactured by Seiko Instruments Co. as a test apparatus, a standard specimen of aluminum, and a specimen container made of aluminum, and under identical conditions including a temperature elevation rate of 15° C./min, in an atmosphere of argon (50 ml/min), and at a specimen weight of 24.5 to 26.5 mg, the obtained profile (μW) of the differential scanning calorimeter measurement is divided by a sample weight and normalized (μW/mg), subsequently a region where the profile of the different scanning calorimeter measurement is horizontal in a section of 0 to 100° C. is defined as a reference level of 0, and the height of the exothermic peak from the reference level is measured.
 2. The high strength aluminum alloy sheet according to claim 1, wherein the aluminum alloy sheet further comprises one or more of 0.5% or less (not including 0%) of Mn, 0.3% or less (not including 0%) of Cr, 0.1% or less (not including 0%) of Zr, 0.1% or less (not including 0%) of V, 0.1% or less (not including 0%) of Ti, 1.0% or less (not including 0%) of Cu, 0.1% or less (not including 0%) of Ag, 0.30% or less (not including 0%) of Zn, and 0.005 to 0.15% of Sn. 